I. Field of the Invention
The present invention relates generally to the modification and control of the temperature of the whole body or a selected body organ. More particularly, the invention relates to a method for controlling whole body or organ temperature by selecting an appropriate gain based on the mass of the body or organ.
II. Description of the Related Art
Organs in the human body, such as the brain, kidney and heart, are maintained at a constant temperature of approximately 37° C. Hypothermia can be clinically defined as a core body temperature of 35° C. or less. Hypothermia is sometimes characterized further according to its severity. A body core temperature in the range of 33° C. to 35° C. is described as mild hypothermia. A body temperature of 28° C. to 32° C. is described as moderate hypothermia. A body core temperature in the range of 24° C. to 28° C. is described as severe hypothermia.
Hypothermia is uniquely effective in reducing brain injury caused by a variety of neurological insults and may eventually play an important role in emergency brain resuscitation. Experimental evidence has demonstrated that cerebral cooling improves outcome after global ischemia, focal ischemia, or traumatic brain injury. For this reason, hypothermia may be induced in order to reduce the effect of certain bodily injuries to the brain as well as other organs.
Catheters have been developed which are inserted into the bloodstream of the patient in order to induce total body hypothermia. For example, U.S. Pat. No. 3,425,419 to Dato describes a method and apparatus of lowering and raising the temperature of the human body. The Dato invention is directed towards a method of inducing moderate hypothermia in a patient using a metallic catheter. The metallic catheter has an inner passageway through which a fluid, such as water, can be circulated. The catheter is inserted through the femoral vein and then through the inferior vena cava as far as the right atrium and the superior vena cava. The Dato catheter has an elongated cylindrical shape and is constructed from stainless steel. By way of example, Dato suggests the use of a catheter approximately 70 cm in length and approximately 6 mm in diameter. However, use of the Dato invention implicates certain negative effects of total body hypothermia.
Due to certain problems associated with total body hypothermia, attempts have been made to provide more selective cooling by intravascularly regulating the temperature of a selected organ. For example, a heat transfer element such as disclosed in application Ser. No. 09/103,342 may be placed in the feeding artery of the organ to absorb or deliver the heat from or to the blood flowing into the organ. The transfer of heat may cause either a cooling or a heating of the selected organ. The heat transfer element is small enough to fit within the feeding artery while still allowing a sufficient blood flow to reach the organ in order to avoid ischemic organ damage. By placing the heat transfer element within the feeding artery of an organ, the temperature of an organ can be controlled without significantly affecting the remaining parts of the body.
The human thermoregulatory system usually maintains a core body temperature near 37° C. but during induced anesthesia, the patient's thermoregulatory defense mechanisms are inhibited. This inhibition lowers the patient's threshold for vasoconstriction and shivering so that the patient losses the ability to control his or her core temperature. In this state of anesthesia, hypothermia can arise from environmental factors, the exposure of body cavities, and the use of active cooling devices. As a result, anesthetized patients are poikilothermic, with body temperatures determined by the environment, over about a 4° C. range of core temperatures.
External cooling/rewarming devices are currently used in surgical procedures to induce hypothermia or to return to normothermic conditions after hypothermia. These devices transport heat flux through the skin, which is an ineffective way to achieve heat transfer because as a result of the different vasoconstrictive states of the patient, blood may not be communicating from the core to the periphery. Endovascular core cooling/rewarming techniques can be much more effective in altering the temperature state of the patient. However, with enhanced effectiveness comes the need to control the degree of heat transfer that is provided to induce, control, and maintain the desired thermal state. Ideally, a heat balance can be achieved by a closed loop feedback system in which the patient's core temperature is sensed and continuously monitored with a standard disposable temperature probe. The temperature is fed back to a controller, which alters the rate of heat transfer through the endovascular catheter, thus achieving the desired temperature state of the patient.
Various feedback control algorithms can be used to control the rate at which heat is extracted from or delivered to the body. In this way the temperature of the body or organ can be varied at a controlled rate and/or maintained at a desired temperature. These algorithms determine the flow rate or temperature of the fluid that is circulated through the catheter based on the temperature history and instantaneous differential between the patient's desired temperature and the patients' actual temperature. The gain of the feedback control system is defined in terms of the power extracted or delivered by the catheter per unit temperature differential between the patient's desired and actual temperature, which is also known as the servo error.
A common feedback control algorithm is incorporated in a PID (proportional-integral-derivative) controller. The parameters used by a PID controller include a gain factor, an integral factor, and a derivative factor to adjust the power transferred by the catheter to control the patients' temperature. An optimal feedback control algorithm should precisely control the patient's temperature by minimizing the system's error in response to a command, i.e. a desired temperature state. Depending on the patients' thermal environment, level of anesthesia, and the surgical manipulations performed on the patient, thermal disturbances are created. The task of the feedback control system is to add or subtract heat from the patient to balance out these thermal disturbances to achieve a neutral heat balance between the patient and his or her environment.
It is important that a proper feedback control gain factor be employed when cooling (or heating) a given body or organ. For example, if the selected gain is too high, the body's response to such a relatively rapid rate of cooling will be to overshoot the target temperature, which may induce a series of damped temperature oscillations about the target temperature. If the gain factor chosen is optimal, the body's response to a thermal disturbance or temperature command (a step change input to the control loop), will yield a response that is critically damped; i.e. in which there is minimal or no temperature overshoot or oscillations. If a low gain level is employed, the system response will be such that a much longer time will be required to achieve the desired steady state temperature value.
One critical parameter used in calculating the appropriate gain factor for the feedback controller is the mass of the body being cooled or heated. Other factors being equal, a greater mass will clearly require a larger gain factor than a smaller mass. Unfortunately, this is not an easy parameter to measure since the mass value that is needed is generally not the actual whole body mass but rather an effective thermal mass that represents a smaller mass volume of the patient. Depending on the degree of vasoconstriction/dilation, the peripheral tissue beds are isolated from the core temperature compartments of the patient, thus reducing the mass that is cooled or heated by over 50%, depending on patient morphology. The optimal gain factor may vary by over a factor of ten from large to small patients with mixed levels of vasoconstriction/dilation. If a single gain factor based on a hypothetical average patient were to be used for all patients, the resulting system response may be very poor for large patients while significantly overshooting the target temperature in small patients.
Accordingly, it would be desirable to provide a method and apparatus for quickly and easily determining a patient's effective thermal mass used to calculate a feedback control gain factor that is employed to control the rate of heat transfer during the induction and maintenance of hypothermia.